Liquid crystals have found use in a variety of electro-optical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them perhaps the most promising candidate materials for non-emissive electro-optical displays available with current technology. Most of these devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the liquid crystal molecules take up a preferred orientation in an applied electric field. Since the coupling to the applied filed by this mechanism is rather weak, the resultant electro-optical response may be too slow for many potential applications. The requirement for speed may become especially important in proportion to the number of elements which have to be addresses in a device. This may result in increasingly impractical production costs for the potential use of such devices in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of ferroelectric liquid crystals (FLCs) of N.A. Clark and S.T. LagerWall (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures prepared using FLC materials having not only high speed response (about 1,000 times faster than currently used twisted nematic devices), but which also exhibit bistable, threshold sensitive switching. Such properties make FLC-based devices excellent candidates for light modulation devices including matrix addresses light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays. A review of the applications of FLC devices is given by S.T. Lagerwall and N.A. Clark (1989) Ferroelectrics 94:3-62.
Tilted smectic liquid crystal phases, in particular smectic C phases, are useful in the preparation of FLC materials. Materials exhibiting such smectic phases which comprise chiral, nonracemic components possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Myer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In FLC devices, such as the Surface Stabilized FLC cells (SSFLC) of Clark and Lagerwall, appropriate application of an external electric field results in alignment of the molecules in the FLC phase with the applied filed. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a wide range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P), and directly proportional to orientational viscosity. Faster switching speeds are thus associated with FLC phases which possess higher polarization density and lower orientational viscosity.
A basic requirement for application of ferroelectric liquid crystals in such devices is the availability of chemically stable LC compounds or mixtures which exhibit chiral tilted smectic phases, preferably chiral smectic C phases, over a substantial temperature range, preferably about room temperature. In some cases, a chiral nonracemic LC material will possess an enantiotropic or monotropic chiral tilted smectic phase. FLC mixtures possessing chiral smectic phases, including those with smectic C* phases (i.e, chiral smectic C), with useful temperature ranges can also be obtained by admixture of chiral, nonracemic compounds, designated FLC dopants, into liquid crystal host material which exhibits a desired tilted smectic phase (an FLC host material) and which may or may not be composed of chiral molecules. Addition of the FLC dopant can affect the ferroelectric polarization density and/or the viscosity of the resultant FLC mixture and thereby affect switching speed. Desirable FLC dopants are molecules which impart high ferroelectric polarization density to an FLC mixture without significantly increasing the orientational viscosity of the mixture. Components of FLC mixtures can also be adjusted to vary phase transition temperatures or ranges.
Other properties of the FLC material, for example the tilt angle of the chiral smectic phase and the birefringence of the material, can affect their usefulness for particular device applications. These properties are affected by the structures of the various components and the amounts of these components in the FLC material. Most effort in the development of FLC materials has been directed toward flat panel display applications. The optimal characteristics for FLC materials used in such displays include high spontaneous polarization (Ps) and low orientational viscosity to achieve fast switching, tilt angles of 22.5.degree. which result in maximum contrast in SSFLC cells switched between crossed polarizers, low birefringence which facilitates construction of a desirable thickness panel and broad temperature range (about room temperature). FLC materials useful in waveguides, integrated optics and spatial light modulators have somewhat different requirements. High polarization and low viscosity are desired for both display and optical switching FLC applications. Enhanced performance in optical switching FLC applications is correlated with high total refractive index change between the switched states which is associated with high birefringence and large tilt angles. A particular type of FLC display device, a dichroic display device containing color switching elements incorporating mixtures of FLCs with dichroic dyes, also requires high tilt FLC material to achieve highest contrast. (See Ozaki et al. (1985) Jpn. J. Appl. Phys. Part I 24 (Suppl. 24-3):63-65.) For applications requiring high tilt angle and/or high birefringence it is desirable to have FLC materials which combine these properties with fast switching speed and broad room temperature smectic C* phases.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively "floppy", i.e., structurally flexible, tails. The tails are typically coupled to the core such that the LC molecule can assume a configuration with relatively linear arrangement of the tails along the long axis of the core. (See Demus et al. (1974) Flussige Kristalle In Tabellen, VEB Deutscher Verlag fur Grundstoffindustrie, Lebzig.) FLC materials have been prepared by introduction of a stereocenter into one of the tails, thus introducing chirality. A variety of FLC materials including materials having phenylbenzoate, biphenyl, phenylpyrimidine, and phenylpyridine core structures have been reported. FLC host materials, having low polarization density or which are achiral, having such core structures have also been reported. FLC host materials typically possess smectic C phases. A number of chiral nonracemic FLC dopant materials are known in the art.
FLC compositions having tilt angles between 30.degree. to 60.degree. have been reported by Ichihashi et al. (1988) EPO publication No. 269,062. The authors infer that tilt angle in the smectic C phase depends on the ordering of liquid crystal phases exhibited by a material, in particular the absence of a higher temperature smectic A phase, is associated with high tilt in the smectic C phase. The reference provides the tilt angles of a number of smectic C phase LCs, no correlation between tilt angle and structure is disclosed. A related EPO application of Furukawa et al. (1988) Publication No. 220,747, refers to a method for controlling tilt angle in FLC smectic C mixtures. The method described involves controlling the tilt angle of a mixture by adjusting the composition of the mixture such that a smectic A phase is present (for low tilt mixtures) or absent (for high tilt mixtures). These reference also refer to a number of components of LC mixtures some of which components have monofluorinated core moieties.
Diacetylenic liquid crystals have been reported by B. Grant (1978) Mol. Cryst. Liq. Cryst. 48:175-182 and E.M. Barrall et al. (1978) Liq. Cryst. Ordered Fluids 3:19-39. Symmetric 4,4'-substituted diphenyldiacetylenes having the general formula: EQU R--Ph--C.tbd.C--C.tbd.C--Ph--R
where R=n-alkyl or n-alkoxy were reported to have liquid crystal properties. The alkyl substituted derivatives were reported to exhibit only nematic liquid crystal phases. No smectic phases were reported with the alkyl derivatives. Most of the alkoxy derivatives similarly exhibited only nematic LC phases, however, the and n-C.sub.14 H.sub.29 O and n-C.sub.15 H.sub.31 O derivatives were reported to display two smectic phases, over a narrow temperature range with the following phase diagrams, respectively: ##STR2## where C=crystal, N=nematic, Sm=smectic and IL=isotropic liquid and temperatures are in .degree. C. Barrall et al. supra reports that Sm.sub.1 appears to be a "tilted smectic B phase" and speculates that Sm.sub.2 is a smectic C phase. Grant supra (1978) also reports the para-substituted phenylacetylenes: EQU R--Ph--C.tbd.C--H, where R is n-alkyl and EQU C.sub.2 H.sub.5 O--CH(CH.sub.3)--O--Ph--C.tbd.C--H
which are employed in the synthesis of the alkyl and alkoxy-substituted diphenyldiacetylenes described.
Gray et al. (1989) WO 89/02425 refers to laterally fluorinated oligophenyls useful as liquid crystals which are biphenyls or terphenyls. Formula III, refers to tolanes, i.e.,: EQU R'--L--C.tbd.C--E--R"
where laterally fluorinated 1,4-benzene rings are included in the listing of L and E. Formula 3.1 refers to a monofluorinated tolane of formula: ##STR3## where R.sub.a and R.sub.b may be alkyl or alkoxy.
Higuchi et al. U.S. Pat. No. 4,728,458 refers to chiral polyphenyl compounds useful in liquid crystal materials. The general formula (1) refers to halogenation of the core moiety which core appears to include tolanes: --Ph--C.tbd.C--Ph--. However, no tolanes appear to be specifically disclosed therein.
Eidenschink et al. WO 87/05018 refers to optically active compounds and the general formula I appears to refer to cores containing halogenated 1,4 phenylene groups and appears to refer to tolane cores. A related application of Krause et al. WO 86/06373 refers in formula I to halogenation of cores containing nitrogen containing heterocycles.
Saito et al. (1988) EP published application 278,665 refers to chiral 2-substituted alkyl ethers useful as components in LC compositions which include those having 3,3'-halogenated biphenyl cores.